Mechanical ventilation appears to upset the normal processes that keep the lungs free of disease. Indeed, ventilator-associated pneumonia (VAP) is reported to be the most common hospital-acquired infection among patients requiring mechanical ventilation. There is a strong correlation between the duration of incubation and development of infection. In a recent large study, the mean interval between incubation and the identification of VAP was 3.3 days. Rello J. et al., “Epidemiology and Outcomes of Ventilator-Associated Pneumonia in a Large US Database” Chest 122:2115 (2002). Importantly, once VAP develops, the patient usually requires a more extended period of ventilation. Unfortunately, prolonging the incubation invites new rounds of deep infection with further decompensation of respiratory function, in a vicious cycle ending frequently in death.
It is well-known to treat such infections with systemically administered antibiotics, but simultaneous treatment of the whole body with multiple antibiotic agents is fraught with complications that range from accelerating the selection of antibiotic-resistant strains to disrupting fluid and electrolyte balance and compromising the antiviral defense mechanisms of mucosal epithelia throughout the body. Systemically administered antibiotics can also have adverse effects on the liver, kidney and skeleton. Such concerns have resulted in a recent call for a de-escalating strategy for antibiotic administration. Hoffken G. and Niederinan M. S., “Nosocomial Pneumonia: The Importance of De-escalating Strategy for Antibiotic Treatment of Pneumonia in the ICU” Chest 122:2183 (2002).
Exacerbating the risks cited above is the fact that the objective of systemic therapy is to achieve high concentrations of antibiotic not in the circulation but on the mucosal side of the bronchi, i.e., in the bronchial secretions. Many antibiotics diffuse poorly from the bloodstream across the bronchi [Pennington, J. E., “Penetration of antibiotics into respiratory secretions,” Rev Infect Dis 3(1):67-73 (1981)], which leads the practitioner to administer higher doses of antibiotic than would be prescribed for a truly systemic infection. Moreover, the purulent sputum that characterizes infected patients tends to compromise the potency of many antibiotics. See e.g., Levy, J., et al., “Bioactivity of gentamicin in purulent sputum from patients with cystic fibrosis or bronchiectasis: comparison with activity in serum,” J Infect Dis 148(6):1069-76 (1983). This factor further motivates the practitioner to prescribe large amounts of antibiotic. These dangers have led some experts to propose that treating lung infections systemically in nosocomial patients should be abandoned. Unfortunately, known alternatives are not attractive either.
An alternative approach in which antibiotics are applied to the oral, gastric and endobronchial mucosa along with systemic administration has been tried. It is very costly and, in any case, is not associated with any ameliorating effect on mortality. It also invites “outbreaks” of antibiotic-resistant infections in intensive care units especially when used indiscriminately.
In another effort to overcome the aforementioned problems associated with systemic administration, various attempts have been made to administer antibiotics directly to the mucosal surface of the lungs of spontaneously breathing patients in aerosols (liquid droplets or dry powders) delivered via various nebulizers. However, more localized administration of antibiotics is controversial. Early studies with aerosolized antimicrobials did not show unambiguously positive results. This may be due, however, to a poor appreciation of the physics of aerosol administration to the intubated patient. It is now recognized that poor system designs and/or improper device usage can result in virtually no aerosol reaching the desired sites in the lungs. “Consensus Statement: Aerosols and Delivery Devices” Respiratory Care 45:589 (2000).
Moreover, even in studies with generally satisfactory results in terms of levels of antibiotic achieved or the reduction in bacterial load observed [Eisenberg, J., et al., “A comparison of peak sputum tobramycin concentration in patients with cystic fibrosis using jet and ultrasonic nebulizer systems. Aerosolized tobramycin study group,” Chest 111(4):955-962 (1997); Ramsey, B. W., et al., “Intermittent administration of inhaled tobramycin in patients with cystic fibrosis. Cystic fibrosis inhaled tobramycin study group,” N Engl J Med 340(l):23-30 (1999)], no effort was made to reduce the amount of antibiotic administered—the nebulizers were charged with quantities of antibiotic equivalent to doses typically administered systemically.
The administration of antibiotics by nebulization in ventilated patients is reportedly even less satisfactory (Fuller, H. D., et al., Pressurized aerosol versus jet aerosol delivery to mechanically ventilated patients. Am. Rev. Respir Dis 1989, 141:440-444; MacIntyre, N., et al., Aerosol delivery to intubated, mechanically ventilated patients. Crit. Care Med 1985, 13:81-84). In ventilated patients, nebulization that bypasses the humidifier and is actuated only on the inspiration phase of the breathing cycle has been attempted using a ventilator (Bear II, Bear Medical Systems, Riverside, Calif.) of obsolete design (Palmer, et al., Crit. Care Med 1998, 26:31-39).
The extreme variability in effective dose that known methods of aerosol delivery engender is not important for conventional drugs such as bronchodilators because of the potency and safety of such agents. Variability is a crucial problem, however, in the case of antibiotics. The risk of pulmonary toxicity discourages the prescription of heroic doses to overwhelm the variability problem. That leaves the patient exposed to the prospect of inadequate treatment, a particularly risky matter. In the worst cases, by the time the insufficiency is recognized, the opportunity to correct the situation is past. In many other cases, the insufficient treatment encourages the selection and growth of antibiotic-resistant organisms in the patient, which totally disarms the practitioner and exposes entire cohorts of patients to danger.
What is needed in the art to encourage the abandonment of systemic antibiotic therapy to treat lung infections in the nosocomial patient is a means of delivering antibiotics directly into the distal airways of the lung. Such means should produce reliably high titers of antibiotics in the bronchial secretions in a short period of time so as to overwhelm all infectious organisms before selection processes can even begin to establish a population of resistant organisms. On the other hand, the invention should provide a reliable means of dose control to avoid. “spillover” into the systemic circulation, pulmonary toxicity, and inadvertent exposure of medical personnel and other patients to escaped antibiotics.